Bluetooth: Add __hci_cmd_sync_ev function

This patch adds a __hci_cmd_sync_ev function, analogous to
__hci_cmd_sync except that it also takes an event parameter to indicate
that the command completes with a special event instead of command
complete. Internally this new function takes advantage of the
hci_req_add_ev function introduced in the previous patch.

The primary expected user of this new function are the setup routines of
HCI drivers which may want to send custom commands and return only when
they have completed.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>